Fluorescent lamp having coated inleads

ABSTRACT

A fluorescent lamp in which the inleads or clamps of the electrodes are coated with a high temperature plastic containing a filler of an insulating inorganic material. The plastic is not removed by baking but remains in place in the completed lamp. One suitable material consists of an aromatic polyimide wherein the filler is zirconium oxide. The coating eliminates or lessens light-absorbing oxide rings and brown patches which form on the envelope ends as a result of the arc taking off from the inleads instead of from the electrode coil, a condition to which instant start lamps are particularly prone.

United States Patent Martyny et al. 1 Dec. 19, 1972 [s41 FLUORESCENTLAMP HAVING 2,542,352 2/1951 Peters ..s1s/1o9 COATED INLEADS PrimaryExaminer-R0 Lake [72] Inventors minim Martyn? Lyndhurstf AssistantExaminer-Jafnes B. Mullins ctor W. Miller, Highland Hts, Ronald L omenwilloughby, a" of AttorneyErnest W. Legree, Henry P. Truesdell, OhioFrank L. Neuhauser, Oscar B. Waddell and Joseph B.

Forman [7 3] Assignee: General Electric Company 22] Filed: Feb. 17, 19717 ABSTRACT A fluorescent lamp in which the inleads or clamps of [21]Appl' n6l70 the electrodes are coated with a high temperature plasticcontaining a filler of' an insulating inorganic [52] US. Cl ..3l3/42,313/108 R, 313/178 i l, The plastic is not removed by baking but [51]Int. Cl .L .1101] 1/02 remains in place in the completed lamp,.()nesuitable Field of Search 108 109, 178, 345 material consists of anaromatic polyimide wherein the filler is zirconium oxide. The coatingeliminates or les- Refefellc Cited sens light-absorbing oxide rings andbrown patches which form on the envelope ends as a result of the arcUNITED STATES PATENTS taking off from the inleads instead of from theelec- 2,885,589 5/1959 Wainio et a1. ..3l3/345 X trode coil, a conditionto which instant start lamps are 3,237,284 3/1966 Bird ..313/345 Xparticularly prone, 3,069,580 12/1962 Waymouth ..3l3/109 2,930,9193/1960 Wainio ..3l3/42 X 7 Claims, 2 Drawing Figures FLUORESCENT LAMPHAVING COATED INLEADS BACKGROUND OF THE INVENTION A problem encounteredwith such lamps is the forw mation of light-absorbing oxide rings andbrown patches on the envelopes at the ends close to the electrodes.Typical brown patches vary in size from A to inch and are locatedopposite the clamps while oxide rings are located k to inch in front ofthe cathode coil. They are caused by the are taking off from the inleador clamp rather than from the electrode coil. Sputtering or vaporizationof the inlead material results which deposits on the envelope wallforming the discoloration. The phenomenon is especially noticeable withinstant-start lamps in which the arc is ignited with cold electrodes bythe application of high voltage. It is generally believed that emissivematerial is vaporized from the filament and deposits upon the inleadsand clamps with the result that at starting, prior to the heating up ofthe filament, there is very little to favor the are taking off from thefilament rather than from the clamp or inlead. When the arc strikes onthe lead, it tends to move about said spark vigorously until finally itsettles on a hot spot on the main body of the coil. In fluorescent lampswherein current is passed through the filamentary electrode to heat itup in order to facilitate starting, the problem of oxide rings and brownspots is not so serious but it does persist.

It has been proposed in the past to cure the brown patch or end bandproblem by coating lead wires, clamps, coil tails and anode wires, ifany, that is, all interior metal parts except the cathode, with arefractory insulating oxide. For instance US. Pat. No. 2,769,112 Heineet al. proposed applying zirconium oxide as a suspension in isopropylalcohol which evaporated completely and left no residue. However, thisscheme proved impractical in commercial production because the coatingwould not adhere well to the metal parts and any breaks in the coatingmake it useless.

Another proposal found in US. Pat. No. 3,069,580 Waymouth et al. is toapply the insulating oxide, for instance aluminum oxide, in a cellulosiclacquer and afterward to remove the cellulose by baking. While thisproposal may work well in the laboratory, it is not suitable forproduction because when the metal parts of the mount are heated to thenecessary temperature in air, excessive oxidation occurs and there is noother practical way of doing it.

SUMMARY OF THE INVENTION start or slimline lamps in which the brown spotproblem has been particularly acute.

In accordance with our invention, suitable plastic materials arearomatic heterocyclic polymers wherein aromatic rings alternate withring-shaped molecules of the parent compound, termed heterocyclic ringsbecause they contain a variety of atoms. Preferred materials arearomatic polyimides consisting of two aromatic rings bracketing aheterocyclic ring which includes a nitrogen atom and two pairs of carbonatoms; Such polyimides are stable for indefinite duration at 300C andhave decomposition points of about 900C. The plastic material isavailable as a varnish and we add thereto a filler consisting of aninorganic insulating oxide such as zirconium oxide. A slurry of thepolyimide-forming varnish and zirconium oxide is made which is appliedto the lead wires by surface methods and which is dried and cured byheating for a short period, for instance 300C for 3minutes. No hightemperature baking to burn out binders'is practiced'and the film remainsextremely flexible so that bending or flexing of the wire does not causethe film to flake off or crack. The film is stable during processing ofthe lamp as well as in operation and does not release gases which woulddeleteriously affect the lamp.

DESCRIPTION OF DRAWING In the drawing:

FIG. 1 shows an instant-start fluorescent lamp in which the invention isembodied.

FIG. 2 is a view to a larger scale of the mount used in the lamp whereinthe inner lead wires including the clamps are coated with a hightemperature plastic polymeric material.

DETAILED DESCRIPTION We have found that high-temperature thermo-settingplastics suitable for the binder in our coating are the polyimidesdescribed in US. Pat. No. 3,179,634 Edwards, Aromatic Polyimides and theProcess for Preparing Them, assigned to DuPont Company. These polyimidesare characterized by a recurring unit having the following structuralformula:

wherein R is a tetravalent radical containing at least one ring of sixcarbon atoms, said ring characterized by benzenoid unsaturation, thefour carbonyl groups being attached directly to separate carbon atoms ina ring of the R radical and each pair of carbonyl groups being attachedto adjacent carbon atoms in a ring of the R radical; and wherein R is adivalent radical containing at least two rings of six carbon atoms, eachring characterized by benzenoid unsaturation, and in which no more thanone of the valence bonds is located on any one of said rings of said Rradical. An example of a polyimide found particularly suitable for theinvention is that of pyromellitic dianhydride and 4,4'-diarnin0 diphenylether. This particular material is commercially available and is sold byduPont de Nemours and Company, Wilmington, Del. under the designationPyre-ML varnish (RX-692). The varnish is actually a solution of acompound convertible to the polyimide solely by heat after application.The polyimide itself is stable over an indefinite duration attemperatures up to 300C and has a decomposition temperature of about900C.

First attempts to use the polyimide as such to provide a protectivecoating on inner lead wires of lamps were unsuccessful. The lead wirenext to thehot spot on the cathode where the arc attaches achieves atemperature of approximately l-200C. At this temperature the plasticfilm was vaporized off the wire. However, when an insulating refractoryinorganic powder such as zirconium oxidewas added to the polyimide as afiller, the result was a coating which remained on the wire throughoutlamp life. i

A suitablemethod of preparing the material and applyingthe coating isdescribed below and uses the following ingredients:

100 ml of Pyre-ML varnish (DuPont) 300 gm of Zirconium Oxide (ZrO,)

Thinner Consisting of N-Methylpyrrolidone and Xylene The varnish and thezirconium oxide are put into a pebble mill and rolled for a period of 24hours during which the zirconium oxide becomes finely dispersed.

Sufficient thinner is added to bring the viscosity of the slurry toapproximately 160 centipoises.

The resulting slurry can be applied to the lead wires by surface methodssuch as brushing, dipping, rolling, etc. After application, the coatingis dried and then cured by heating for a period depending upon thetemperature, for example, 3 minutes at 300C. At lower temperatures, alonger time is required and the-period may be shortened if a highertemperature is used.

While the formulation given above is preferred, the proportion ofzirconium oxide to varnish may be varied withinratherwide limits, forinstance from about 100 to 500 grams for the same quantity of polyimideforming varnish. Also other refractory inorganic insulating materialsmay be used as fillers, for instance aluminum oxide and magnesium oxide.

A lamp embodying the invention is illustrated in FIG. 1 comprising aglass tube 1 carrying a phosphor 2 on its interior surface andcontaining an ionizable filling including mercury and an inert startinggas such as argon in the usual manner. A stem 3 such as illustrated inFIG. 2, is sealed to each end of the tube to close it off and lead-inwires 4,5 are sealed through the stem. Extemally of the tube, thelead-in wires 4,5 are connected electrically to the single pin 6 whichprojects outwardly from the insulating base shell 7 cemented to the tubeend. A coiled coil filament 8 of tungsten wire filled and coated withelectron-emitting oxides including barium oxide (represented byspeckling) is supported near its ends by the lead-in wires 4,5 whoseends 9 are bent back on themselves to clamp the filament. The lead-inwires including the clamps are covered by a coating 10 of the polyimideplastic containing zirconium oxide. The coating may extend from thepoint where the leadin wires emerge from the glass of the press, or itmay extend only from the last bend in the inlead, as shown in thedrawing. It should include the entire clamp portion in which thefilament is seized. The coating 10 overruns the end portions 11 of thefilament outside the space between the lead-in wires 4,5, that isextending beyond the clamps 9. However, the plastic coating ingoperation. The plastic coating insulatesv from the discharge allportions of the electrode and associated metal parts lying as close ornearly as close to the other electrode as the emissive portion of theelectrode, and prevents the arc from taking off from any other placethan the emissively coated filament. r

The major advantage which the material in, ac-, cordance with ourinvention has over others proposed inthe past is that it avoids the needfor high temperature baking to-bum out the binders previously used. Alsothe film is extremely flexible so that bending or flexing of the wiredoes not cause the coating to flake off or crack. The polyimide is verystable while the lamp is operating and there is, no release of gaseousmaterials into the lamp atmosphere which would deleteriously affect thelamp. We have theorized that immediately next to the hot spot the leadis sufficiently insulated by the zirconium oxide, while at distancesmore remote from the hot spot where the temperature falls below 900C,the insulation is provided by both the polyimide coating and thezirconium oxide dispersed in it.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

l. A low pressure electric discharge lamp comprising a vitreous envelopecontaining an ionizable medium and filamentary electrodes in the endsthereof mounted on inlead wires sealed through the envelope,

each electrode comprising a refractory metal wire coil supported betweensaid lead wires, the major portion of said coil being filled or coatedwith elecwherein R is a tetravalent radical containing at least one ringof six carbon atoms, said ring characterized by benzenoid unsaturation,the four carbonyl groups being.

attached directly to separate carbon atoms in a, ring of the R radicaland each pair of carbonyl groups being attached to adjacent carbon atomsin a ring of the R radi-- wherein R is a tetravalent radical containingat least filler ring of six carbon atoms, said ring characterized bybenzenoid unsaturation, the four carbonyl groups being attached directlyto separate carbon atoms in a ring of the R radical and each pair ofcarbonyl groups being attached to adjacent carbon atoms in a ring of theR radical; and wherein R is a divalent radical containing at least tworings of six carbon atoms, each ring characterized by benzenoidunsaturation, and in which no more than one of the valence bonds islocated on any one of said rings of said R radical and wherein theinorganic filler is zirconium oxide or aluminum oxide or magnesiumoxide.

6. A lamp as in claim 1 wherein the polymeric plastic is the polyimideof pyromellitic dianhydride and 4,4- diamino diphenyl ether andwhereinthe inorganic filler is zirconium oxide, or aluminum oxide or magnesiumoxide.

7. A lamp as in claim 1 wherein the polymeric plastic is the polyimideof pyromellitic dianhydride and 4,4- diamino diphenyl ether and whereinthe inorganic filler is zirconium oxide provided in a ratio of to 500grams of filler to 100 milliliters of polyimide-forming varnish.

l060ll 0196

2. A lamp as in claim 1 wherein the polymeric plastic is an aromaticpolyimide.
 3. A lamp as in claim 1 wherein the polymeric plastic is anaromatic polyimide characterized by a recurring unit having thefollowing structural formula:
 4. A lamp as in claim 1 wherein thepolymeric plastic is the polyimide of pyromellitic dianhydride and4,4''-diamino diphenyl ether.
 5. A lamp as in claim 1 wherein thepolymeric plastic is an aromatic polyimide characterized by a recurringunit having the following structural formula:
 6. A lamp as in claim 1wherein the polymeric plastic is the polyimide of pyromelliticdianhydride and 4,4''-diamino diphenyl ether and wherein the inorganicfiller is zirconium oxide, or aluminum oxide or magnesium oxide.
 7. Alamp as in claim 1 wherein the polymeric plastic is the polyimide ofpyromellitic dianhydride and 4,4''-diamino diphenyl ether and whereinthe inorganic filler is zirconium oxide provided in a ratio of 100 to500 grams of filler to 100 milliliters of polyimide-forming varnish.